An atmospheric pressure ionization interface is used to ionize and mass analyze a liquid sample or components to be analyzed in an eluate which have been separated by a liquid chromatograph. Typical and known atmospheric pressure ionization methods include an electro spray ionization (ESI) method and an atmospheric pressure chemical ionization (APCI) method. Generally, such an atmospheric pressure ionization interface is often used in combination with a quadrupole mass spectrometer, an ion trap mass spectrometer, or a time-of-flight mass spectrometer.
A characteristic of an atmospheric pressure ionization interface, particularly an ESI interface, is that it tends to generate a multivalent ion or ions having a plurality of electric charges in the ionization process of a target compound. A multivalent ion is advantageous that the range of the m/z values to be analyzed can be restricted to a relatively low range since the m/z value of a multivalent ion becomes smaller according to its valence than the molecular weight of its original compound. In particular, in analyzing a compound having a large molecular weight such as a protein or a peptide, despite that the m/z value of a monovalent ion can exceed the measurable range of a mass spectrometer, the use of a multivalent ion can bring the m/z value to the measurable range of the mass spectrometer. Therefore, a mass analysis using a multivalent ion is very effective in identifying a compound having a large molecular weight.
Naturally, in mass analyzing a compound having a large molecular weight, peaks originating from ions of a variety of valences appear on a mass spectrum. Also, in analyzing a sample in which various kinds of compounds are mixed, peaks originating from the respective compounds are mixed on the mass spectrum. Hence, the data analysis for such a mass spectrum is complicated. The method of separating and extracting the peak of the target compound from a mass spectrum on which a plurality of multivalent ion peaks are observed and then obtaining its m/z value is called deconvolution (refer to Non-Patent Document 1 and other documents).
In the course of an ionization by the ESI method or other method, a variety of ions are added to or desorbed from the target compound to generate a multivalent ion or ions. For example, in a cation measurement mode, other than a proton-added ion in which one proton (H+) has been added to the target compound, adduct ions can be detected in which a variety of components such as ions existing in the mobile phase used in a liquid chromatograph and ions from the metal of the piping, e.g. sodium (Na), ammonia (NH4), or both a proton and methanol, are added to the target compound. Meanwhile, in an anion measurement mode, in addition to a proton-desorbed ion, in which one proton has been desorbed from the target compound, adduct ions are detected in which the components of acetic acid (CH3COOH), formic acid (HCOOH), or other element in the mobile phase are added to the target compound.
Adduct ions having the same valence may have different m/z values due to the substance which has been added to or desorbed from the target compound. Therefore, in order to perform a deconvolution process to a mass spectrum on which peaks of a multivalent ion or ions appear, it is necessary to determine what component has been added to or desorbed from the target compound. For this purpose, conventionally a deconvolution process as described in Patent Document 1 and other documents has been performed in the following procedure. First, before performing an analysis operation, a user enters the kind of the component (or ion) which is added to or desorbed from the target compound in the ionization process. In response to this input, a data analysis processor collects a plurality of peaks originating from components having the same mass M, by using the fact that the m/z values of the peaks of the multivalent ions observed on a mass spectrum present an orderly series in which the relation (M/n)−A, i.e. the combination of n and M, always holds, where n is a natural number, A is the mass (or m/z value) of the added ion, and M is the mass of the target compound.
However, the kind and the tendency of occurrence of an ion addition reaction or an ion desorption reaction with a compound as previously described vary depending on the properties of the compound, the conditions of the ionization, and other factors. Further, controlling such an ion addition reaction or ion desorption reaction is difficult. Therefore, knowing beforehand what kind of adduct ions will be detected is a considerably difficult task. Since such a task requires a compilation of knowledge and experience, such an analytical operation is usually assigned to an analysis operator having a high skill, and the problem is that a person who has a limited knowledge or experience cannot perform an accurate analysis. In addition, even when a skilled analysis operator performs an analytical operation, a certain amount of trial-and-error operation is required, which disadvantageously elongates the operation and decreases the throughput.
Furthermore, in analyzing a sample in which a variety of compounds are mixed, a large number of peaks originating from the plurality of compounds are observed on the mass spectrum. This might inadvertently cause an incorrect setting of valence n, leading to an incorrect final mass calculation.
[Patent Document 1] U.S. Pat. No. 5,130,538
[Non-Patent Document 1] “(Technical Classification) 2-4-1-4 General Techniques of Mass Analysis/Data Processing/Spectrum Processing/Deconvolution,” (online), Japanese Patent Office, (Search Date: May 1, 2010), Internet <http://www.jpo.go.jp/shiryou/s_sonota/hyoujun_gijutsu/mass/2-4-1.pdf>